Disneyland is not the place for it.
I adore Disneyland, but I really wish it had more open areas and areas designed for kids and families to run around, lay in some shade, and just relax. Did I say "more"? I could revise that to "any." Disneyland does not have places to let your kids burn off their energy, or just unwind, not really. Well, I guess Tom Sawyer Island qualifies - and is the best option, but still not great due to crowds and the trails are not necessarily safe to let little ones nagivate alone. (Disneyland and littles ones - never leave 'em unattended.)
California Adventure has a couple decent options for running around and cooling off. Since the park is less crowded than Disneyland, in general, it is a better place to relax and get some space between you and thousands of others. Our kids - OK and me, too - loved the Princess Dot Puddle Park. It's in A Bug's Land - nice vegetation - leaves taller than my kids - and its like playing in the sprinklers. This is a great place to cool down! My two kids and I got totally soaked. We brought a change of clothes for them. Not for me. I walked around in wet clothes and underwear (Not so comfy!) for about an hour until I dried off. Heck - in August I think my shirt was dry in about 15 minutes!
The other really good place to let kids be kids is in The Redwood Creek Challenge Trail. There are trails and bridges and lots of shaded room to run around in. Even during busy days, this area doesn't seem to get too crowded. It is also very close to a major attraction - The Grizzly River Run. So, if part of your family is too short to ride (42" height requirement), or doesn't like getting wet - this is a good place for them to hang out while the taller and more adventurous do the River Run.
Special note: There is a single rider option on Grizzly River Run. Two adults who both want to go on the ride but someone has to watch the kids? Leave one adult with the kids at the Redwood Creek Trail, the other goes up the exit stairs and tells the cast member on the loading platform they are a single rider. They get loaded in a boat with some others, almost immediately. That way, the boat doesn't go down only partially loaded and some lucky person didn't have to wait in line for an hour or more. (Sometimes there is a single rider ticket near the fast pass line. Look for a cast member near the entrance to the attraction and inquire about it. If you can't find one - you are probably OK to enter without one and the cast member on the loading platform can tell you where to get one next time. Just be polite. They deal with a lot of people.) After the first person goes, trade places with the adult who was just playing with the kids on Redwood Creek Trail.
